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D. J. RANDALL
aorta. There is hardly any backflow in the ventral aorta as the valves
close, but if conal systole is prevented, the proximal (lower) and middle
valves are incompetent throughout the cardiac cycle and there is a large
backflow of blood in the ventral aorta as the distal valves close. The
conus is within a rigid pericardium, and the subatmospheric intrapericardial pressures tend to dilate the conus and increase the incompetence
of the conal valves. If the intrapericardial pressure is experimentally reduced, the backflow in the ventral aorta increases as the distal valve
closes. Thus, Satchell and Jones (1967) suggest that conal systole plays
no part in ejecting blood into the ventral aorta but maintains the competence of the proximal and middle conal valves for a short period of
time subsequent to the ventricular ejection of blood. The suggestions
made by Satchell and Jones (1967) are not mutually exclusive. Conal
systole could play a part in ejecting blood into the ventral aorta and
also maintain the competence of the conal valves. Sudak ( 1965a), Johansen et al. (1966), and Hanson (1967) have reported data that either indicate or demonstrate enhanced flow in the ventral aorta that can be
ascribed to conal systole. Thus, in most elasmobranchs studied, conal
systole maintains flow in the ventral aorta during ventricular relaxation.
A series of conal valves are required because the conduction velocity
of the wave of excitation over the conus is relatively slow (Tebecis,
1967). The number of sets of valves present is presumably related to the
length of the conus and the velocity of conduction. A series of valves
is required if proximal portions of the conus relax, and the valves become
patent, while systole is still occurring in more distal portions of the
conus. Thus a series of valves permits the progressive contraction and
relaxation of the conus and reduces any backflow resulting from conal
systole.
Work done by the contracting ventricle and the efficiency of contraction have not been calculated for any fish heart. Work done is force
times distance moved, which, in terms of the heart, is pressure times
volume ejected. Pressure and flow continually change during ventricular
systole. Work is done only during periods when blood is ejected from
the heart. A measure of ventricular stroke work can be obtained by
plotting ventricular pressure against ventricular volume ( Fig. 6). The
area of the enclosed loop is a measure of the stroke work of the ventricle
(see Rushmer, 1961). Although ventricular pressures have been measured, there are no reported data in the literature of associated volume
changes in the ventricle during systole, or indeed in any other part of
the heart. In Fig. 6, changes in ventricular volume were calculated from
increments of ventral aortic flow. This is not an accurate assessment of
volume change in the ventricle because, during systole, the volume of
D. J. RANDALL
aorta. There is hardly any backflow in the ventral aorta as the valves
close, but if conal systole is prevented, the proximal (lower) and middle
valves are incompetent throughout the cardiac cycle and there is a large
backflow of blood in the ventral aorta as the distal valves close. The
conus is within a rigid pericardium, and the subatmospheric intrapericardial pressures tend to dilate the conus and increase the incompetence
of the conal valves. If the intrapericardial pressure is experimentally reduced, the backflow in the ventral aorta increases as the distal valve
closes. Thus, Satchell and Jones (1967) suggest that conal systole plays
no part in ejecting blood into the ventral aorta but maintains the competence of the proximal and middle conal valves for a short period of
time subsequent to the ventricular ejection of blood. The suggestions
made by Satchell and Jones (1967) are not mutually exclusive. Conal
systole could play a part in ejecting blood into the ventral aorta and
also maintain the competence of the conal valves. Sudak ( 1965a), Johansen et al. (1966), and Hanson (1967) have reported data that either indicate or demonstrate enhanced flow in the ventral aorta that can be
ascribed to conal systole. Thus, in most elasmobranchs studied, conal
systole maintains flow in the ventral aorta during ventricular relaxation.
A series of conal valves are required because the conduction velocity
of the wave of excitation over the conus is relatively slow (Tebecis,
1967). The number of sets of valves present is presumably related to the
length of the conus and the velocity of conduction. A series of valves
is required if proximal portions of the conus relax, and the valves become
patent, while systole is still occurring in more distal portions of the
conus. Thus a series of valves permits the progressive contraction and
relaxation of the conus and reduces any backflow resulting from conal
systole.
Work done by the contracting ventricle and the efficiency of contraction have not been calculated for any fish heart. Work done is force
times distance moved, which, in terms of the heart, is pressure times
volume ejected. Pressure and flow continually change during ventricular
systole. Work is done only during periods when blood is ejected from
the heart. A measure of ventricular stroke work can be obtained by
plotting ventricular pressure against ventricular volume ( Fig. 6). The
area of the enclosed loop is a measure of the stroke work of the ventricle
(see Rushmer, 1961). Although ventricular pressures have been measured, there are no reported data in the literature of associated volume
changes in the ventricle during systole, or indeed in any other part of
the heart. In Fig. 6, changes in ventricular volume were calculated from
increments of ventral aortic flow. This is not an accurate assessment of
volume change in the ventricle because, during systole, the volume of